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Last updated on 2024年6月8日

How do you model turbulent flow accurately in engineering simulations?

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Modeling turbulent flow, a phenomenon where fluid motion is chaotic and unpredictable, is a cornerstone of engineering design. Accurate simulations of turbulent flow are critical for the development of various systems, from aircraft to pipelines. While turbulence is complex, understanding the principles behind its simulation can help you grasp how engineers tackle this challenge. The key lies in using the right mathematical models and computational techniques to predict how fluids will behave under different conditions. This article will delve into the strategies used to model turbulent flow accurately, ensuring that your engineering simulations reflect real-world scenarios as closely as possible.

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Rajat Walia

CFD Engineer @ Mercedes-Benz | Aerodynamics | Thermal | Aero-Thermal | Computational Fluid Dynamics | Valeo | Formula…

1 Understanding Turbulence

Turbulence is characterized by irregular, chaotic fluid movement, which differs from the orderly pattern of laminar flow. It's essential in engineering simulations to predict phenomena like drag, heat transfer, and chemical reactions. To model turbulence, you need to understand the Reynolds number, which predicts the flow regime based on fluid properties and flow conditions. When the Reynolds number exceeds a certain threshold, the flow becomes turbulent, necessitating more complex simulation approaches to capture the erratic motions of the fluid particles.

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Rajat Walia

CFD Engineer @ Mercedes-Benz | Aerodynamics | Thermal | Aero-Thermal | Computational Fluid Dynamics | Valeo | Formula Student
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It is very important to understand turbulent flows physics, turbulent boundary layers, and their modeling. Homogeneous turbulence, isotropic turbulence, wall-bounded turbulence, and free shear flow; such flows exhibit unique features of fluid flow behavior. Having a sound understanding of different types of turbulent flow would help us to judge whether the selected turbulence model for your CFD simulation is able to model the unique features of these different turbulent flow physics or not.

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Kumar Abhinav

Mechanical Engineering l Birla Institute of Technology and Science , Pilani
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Major issues include the wide range of scales, high computational costs, near-wall modeling difficulties, and limitations of turbulence models. Accurate boundary conditions, grid dependence, and validation further complicate the process. Balancing accuracy and computational feasibility often requires hybrid approaches and advanced modeling techniques.

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2 Reynolds-Averaged Navier-Stokes

The Reynolds-Averaged Navier-Stokes (RANS) equations are the foundation of many turbulence models. These equations simplify the chaotic fluctuations of turbulent flow by averaging the flow variables over time. While RANS models are less computationally intensive, they may not capture all the nuances of highly turbulent flows or accurately predict flow separation. However, for many engineering applications, RANS provides a balance between accuracy and computational feasibility, making it a popular choice for steady-state simulations.

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Rajat Walia

CFD Engineer @ Mercedes-Benz | Aerodynamics | Thermal | Aero-Thermal | Computational Fluid Dynamics | Valeo | Formula Student
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It is very important to understand that we apply Reynolds averaging to obtain Reynolds-averaged Navier-Stokes equations. While performing Reynolds averaging on the instantaneous flow quantities, we average out all the information about turbulence. This is a very strong simplification. This also leads to closure models because RANS equations are not closed. Somehow, we have to bring back the effect of turbulent fluctuating quantities on the mean flow. For that, we model the "Reynolds stress" term, which is a second-order tensor, using various 0, 1, 2, or 7 equation turbulence models.

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Kumar Abhinav

Mechanical Engineering l Birla Institute of Technology and Science , Pilani
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RANS is essential in CFD for simulating turbulent flow due to its efficiency and practicality, despite some accuracy limitations in complex flows. It remains widely used in industry, with advanced models and hybrid approaches improving its effectiveness in capturing key turbulence features.

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3 Large Eddy Simulation

When your project requires more detailed turbulence modeling, Large Eddy Simulation (LES) is a powerful technique. LES resolves large-scale turbulent structures while modeling smaller eddies statistically. This approach requires more computational power than RANS but offers a more accurate depiction of turbulence for complex flows. LES is particularly useful for unsteady simulations where capturing transient effects is crucial for predicting system performance.

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Kumar Abhinav

Mechanical Engineering l Birla Institute of Technology and Science , Pilani
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Managing Large Eddy Simulation (LES) involves: understanding LES principles, using fine grid resolution, selecting appropriate subgrid-scale models, applying suitable boundary conditions, employing implicit time integration schemes, initializing turbulence properly, conducting thorough post-processing, validating results, optimizing for efficiency, and iteratively improving the approach.

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4 Direct Numerical Simulation

For the most accurate turbulence modeling, Direct Numerical Simulation (DNS) solves the Navier-Stokes equations without any turbulence modeling, capturing all scales of motion. DNS provides detailed insights into the turbulent flow but at the cost of significant computational resources. It's typically reserved for fundamental research or very small-scale engineering problems where precision is paramount and computational expense can be justified.

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Kumar Abhinav

Mechanical Engineering l Birla Institute of Technology and Science , Pilani
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Direct Numerical Simulation (DNS) in turbulent simulation means using computers to solve all the details of turbulent flows without simplifications. It gives a clear picture of how turbulence works but requires a lot of computing power and is mostly used for simple cases. DNS helps understand turbulence better and test other turbulence models.

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5 Hybrid Approaches

Hybrid approaches combine RANS and LES methodologies to offer a middle ground between computational efficiency and accuracy. Techniques like Detached Eddy Simulation (DES) use RANS in regions with stable flow and switch to LES in areas with significant turbulence. This allows for detailed turbulence modeling where it's most needed while keeping computational costs lower than a full LES or DNS would require.

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Kumar Abhinav

Mechanical Engineering l Birla Institute of Technology and Science , Pilani
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Hybrid approaches in turbulent simulation mix different methods to get a good balance between accuracy and how much computer power is needed. They use a detailed but slow method in important areas and a simpler but faster method in less important areas. This helps save time and computer resources while still getting accurate results for tricky turbulent flows.

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6 Turbulence in Practice

In practical engineering applications, you'll often use commercial Computational Fluid Dynamics (CFD) software that includes a range of turbulence models. Choosing the right model depends on your project's specific needs, considering factors such as the level of detail required, available computational resources, and the nature of the turbulent flow. It's vital to validate your simulation results against experimental data or established benchmarks to ensure accuracy.

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Kyle Nyberg

Cheif Engineer / Owner at KTN Thermodynamics
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Utilizing Solidworks you first need to understand what you are trying to measure and simulate, then rapid prototype your design to initiate real world testing that way you know if your results are accurate

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7 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?

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How do you model turbulent flow accurately in engineering simulations?

1

2

3

4

5

6

7

1 Understanding Turbulence

2 Reynolds-Averaged Navier-Stokes

3 Large Eddy Simulation

4 Direct Numerical Simulation

5 Hybrid Approaches

6 Turbulence in Practice

7 Here’s what else to consider

Engineering Design

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How do you model turbulent flow accurately in engineering simulations?

1

2

3

4

5

6

7

1 Understanding Turbulence

2 Reynolds-Averaged Navier-Stokes

3 Large Eddy Simulation

4 Direct Numerical Simulation

5 Hybrid Approaches

6 Turbulence in Practice

7 Here’s what else to consider

Engineering Design

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